Buffer.isEncoding and string_decoder.normalizeEncoding shared
quite a bit of logic. This moves the primary logic into
internal/util. The userland modules that monkey patch Buffer.isEncoding
should still work.
PR-URL: https://github.com/nodejs/node/pull/7207
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>